1. Field of the Invention
This invention relates to fuel cell technology and more particularly to a solid oxide fuel cell capable of being operated with a sulfur-containing hydrocarbon fuel, as well as methods of fabrication of such fuel cells and their use for producing electric energy.
2. Description of Prior Art
Solid-oxide fuel cells (SOFCs) have grown in recognition as a viable, high temperature source of electric energy. As the operating temperatures of these fuel cells typically exceed 600° C. and may be as high as 1,000° C., the materials used for the cell components are limited to those that are stable at such temperatures. The electrolyte of the cells is made primarily from dense ceramic materials. The electrolyte conducts oxygen anions (O2−) but is an electronic insulator.
It is known to prepare anode materials for solid-oxide fuel cells using nickel (Ni) cermets prepared from NiO and yttria-stabilized zirconia (YSZ) powders. N. Q. Minh, Journal of the American Ceramic Society, 76: 563 (1993), the entire disclosure of which is incorporated by reference herein. High-temperature calcination at greater than 1200° C. is essential in order to obtain the necessary ionic conductivity in the YSZ portion of the anode.
In most conventional fuel cells, hydrogen gas is either fed directly to the anode or produced by steam reforming of hydrocarbons, such as methane. Steam reforming is costly and adds significant complexity to the system. It has been proposed to use dry methane; however, Ni catalyzes the formation of carbon fibers in dry methane, resulting in carbon formation on the anode. Ni-containing anodes can be used only if the fuel cells incorporating them are operated at steam/methane ratios greater than 1.7. However, there are significant advantages to be gained by operating under dry conditions. These include easier management of heat, no requirement of adding steam, and the possibility of lower operating temperatures due to the fact that equilibrium for direct oxidation is always favorable.
Some of the shortcomings of nickel-based anode materials in SOFCs have been overcome by the use of copper-containing anodes. See, for example, R. Gorte et al., Advanced Materials, 12: 1465-69 (2000), the entire disclosure of which is incorporated by reference herein. It has been shown that replacement of nickel with copper in the anode material avoids the problem of carbon formation, when using dry hydrocarbon fuels. See S. Park, et al., Nature, 404: 265-57 (2000), the entire disclosure of which is incorporated by reference herein.
Fuel cells are normally operated with hydrogen as the fuel. It has been proposed to replace hydrogen with commercially available and more economical hydrocarbon fuels such as natural gas, gasoline, diesel fuel, naphtha, fuel oil and the like. Such raw fuels are not currently in use as a fuel source suitable for a fuel cell because these fuels contain relatively high levels of sulfur, often as naturally-occurring complex organic sulfur compounds. For example, gasoline sold in the United States has an average sulfur level of 300 ppm. Also, sulfur compounds such as mercaptans and thiols are added as odorants to natural gas at levels between 10 and 20 ppm so that leaks may be detected.
Most conventional fuel cells are operated on hydrogen gas. Oxidation in the presence of sulfur results in a poisoning effect on catalysts used in the hydrogen generation system, often including the fuel cell anode catalyst. Accordingly, the hydrocarbon fuels currently in use in fuel cells are routinely desulfurized and then reformed to hydrogen gas.
In one such operation, conventional fuel-processing systems used with stationary fuel cell power plants include a thermal steam reformer, such as that described in U.S. Pat. No. 5,516,344. In such a fuel-processing system, sulfur is removed by conventional hydrodesulfurization techniques, which typically rely on a certain level of recycle as a source of hydrogen for the process. The recycle hydrogen combines with the organic sulfur compounds to form hydrogen sulfide within a catalytic bed. The hydrogen sulfide is then removed, using a zinc oxide bed to form zinc sulfide. The general hydrodesulfurization process is disclosed in detail in U.S. Pat. No. 5,292,428. While this system can be used in large stationary applications, it adds significant complexity to the systems.
Other fuel-processing systems, such as conventional auto-thermal reformers, which have a higher operating temperature than conventional thermal steam reformers, can produce hydrogen-rich gas in the presence of the aforesaid complex organic sulfur compounds without prior desulfurization. According to U.S. Pat. No. 6,159,256, when using an autothermal reformer to process raw fuels containing complex organic sulfur compounds, the result is a loss of autothermal reformer catalyst effectiveness and useful catalyst life of the remainder of the fuel-processing system. Before feeding the reformate to the fuel cell, it has been reported that the H2S concentration must be decreased to 0.05 ppm. Y. Matsuzaki and I. Yasuda, “SOFC VII, Proceeding of the 7th Intern. Symp.,” Electrochemical Society, Pennington, N.J., 2001:16 (2001), p. 769.
Alternatively, sulfur, in the form of hydrogen sulfide, can be removed from the gas stream by passing the gas stream through a liquid scrubber, such as sodium hydroxide, potassium hydroxide or amines. Liquid scrubbers are large and heavy and are, therefore, useful principally only in stationary fuel cell power systems.